A hydraulic drive system for a hydraulic working machine having a swing structure, such as hydraulic excavators, includes a swing motor unit that has a hydraulic swing motor for driving the swing structure. The swing motor unit has a relief valve, called an overload relief valve, disposed thereinside. The relief valve is adapted to limit a supply pressure of a hydraulic fluid to or a discharge pressure thereof from the hydraulic swing motor in order to prevent the pressure from exceeding a predetermined level.
A relief valve having a shockless function to relieve a starting or stopping shock of swinging operations, as described in Patent Document 1, is known as an example of a relief valve used in a hydraulic swing motor.
Also known is a hydraulic drive system that enables, as described in Patent Document 2, a previously set pressure of a relief valve in a hydraulic swing motor, that is, a relief pressure, to be changed. Making the relief pressure changeable enables a driving pressure or braking pressure of the hydraulic swing motor to be changed and a maximum driving torque or braking torque thereof to be changed. According to Patent Document 2, the previously set pressure of the relief valve is changed according to a particular posture of a front work implement, and as an inertial mass of an upper swing structure changes according to the particular posture of the front work implement, the maximum driving torque or braking torque of the hydraulic swing motor is changed, thereby improving operability.